Method of measuring concentration of fe in p-type silicon wafer and spv measurement apparatus

ABSTRACT

Provided is a method of measuring the concentration of Fe in a p-type silicon wafer by an SPV method enabling improvement in the measurement accuracy for Fe concentrations of 1×10 9 /cm 3  or less. The method of measuring the concentration of Fe in a p-type silicon wafer includes measuring an Fe concentration in the p-type silicon wafer based on measurement using an SPV method. The measurement is performed in an atmosphere in which the total concentration of Na + , NH 4   + , and K +  is 1.750 μg/m 3  or less, and the total concentration of F − , Cl − , NO 2   − , PO 4   3− , Br − , NO 3   − , and SO 4   2−  is 0.552 μg/m 3  or less.

TECHNICAL FIELD

This disclosure relates to a method of measuring the concentration of Fein a p-type silicon wafer by a surface photovoltage (SPV) method and toan SPV measurement apparatus.

BACKGROUND

Contamination of a p-type silicon wafer with Fe adversely affects thecharacteristics of a device fabricated using the wafer. With this beingthe case, techniques to evaluate the concentration of Fe in p-typesilicon wafers in a simplified manner have been developed. One of thetechniques is a known method of determining the concentration of Fe in ap-type silicon wafer using the results of electrically measuring thediffusion length of minority carriers by the SPV method.

In the SPV method, a p-type silicon wafer is illuminated with lights ofcertain wavelengths, and the surface photovoltage (SPV signal) of thewafer of that time is measured to determine the diffusion length ofminority carriers in the wafer. This procedure is hereinafter alsoreferred to as simply “SPV measurement”. The SPV method is an excellentmethod that enables a shorter measurement time as compared with othermethods and allows for non-contact and non-destructive measurements.

SPV measurements are known to involve two types of measurement modes:Standard Mode and Ultimate Mode. In the SPV method, a plurality oflights of different wavelengths are necessarily used to perform the SPVmeasurements. Standard Mode is a typical method of performing an SPVmeasurement using a certain wavelength and then successively performingSPV measurements using wavelengths that are different from theproceeding wavelength and from each other. Ultimate Mode is a specialmethod of performing SPV measurements at a time by concurrently castinglight of a plurality of different wavelengths.

PTL 1 describes a technique of measurements with a lowered detectionlimit for the Fe concentration in a short time by performing an SPVmeasurement in Ultimate mode and by controlling three measurementparameters of Time Between Readings, Time Constant, and Number ofReadings to given numerical ranges.

CITATION LIST Patent Literature

PTL 1: WO 2017/061072 A

SUMMARY Technical Problem

The environment in which an SPV measurement apparatus for performingsuch SPV measurements is placed is recommended to meet: temperature:24±2° C., relative humidity: 30% to 50%, and cleanliness: class 7 (JISstandards), as described in the specifications of typical SPVmeasurement apparatuses provided by the manufacturers. Conventionally,SPV measurements have been typically performed in the above recommendedenvironments. The inventors of this disclosure however recognized thefollowing problems.

That is, as long as an SPV measurement is performed in the aboverecommended environment, sufficient measurement accuracy has beenachieved in quantifications at Fe concentrations of the order of1×10⁹/cm³ or the order of 1×10¹⁰/cm³. However, it was found that inquantifications at Fe concentrations of 1×10⁹/cm³ or less, even if SPVmeasurements were performed in the above recommended environment, themeasured values of the same wafer subjected to SPV measurements aplurality of times varied; in other words, sufficient measurementaccuracy was not achieved. For the conventional recommended environment,only the temperature, humidity, and the air particles (cleanliness) wereconsidered, and conditions other than those are not specified. Further,in PTL 1, the environment in which an SPV measurement apparatus isplaced is not considered in any way.

In view of the above problems, it could be helpful to provide a methodof measuring the concentration of Fe in a p-type silicon wafer by an SPVmethod enabling improvement in the measurement accuracy for Feconcentrations of 1×10⁹/cm³ or less and an SPV measurement apparatus.

Solution to Problem

With a view to solving the above problems, the inventors of thisdisclosure diligently studied to find ways to improve the measurementaccuracy for Fe concentrations in a low concentration region of, forexample, 1×10⁹/cm³ or less by optimizing the environment in which an SPVmeasurement apparatus for performing an SPV measurement is placed interms other than temperature, humidity, and air particles (cleanliness).As a result, the inventors first found that the ion concentration in theenvironment in which the SPV measurement apparatus was placed affectedthe measurement accuracy for the Fe concentration, and the measurementaccuracy for Fe concentrations of 1×10⁹/cm³ or less could be improved bycontrolling the ion concentration to a certain value or lower. Further,the inventors secondly found that the organic concentration in theenvironment in which the SPV measurement apparatus was placed affectedthe measurement accuracy for the Fe concentration, and the measurementaccuracy for Fe concentrations of 1×10⁹/cm³ or less could be improved bycontrolling the organic concentration to a certain value or lower.

A first group of inventions of this disclosure completed based on theabove findings primarily includes the following features.

(1) A method of measuring a concentration of Fe in a p-type siliconwafer based on measurement by an SPV method,

-   -   wherein the measurement is performed in an atmosphere in which a        total concentration of Na⁺, NH₄ ⁺, and K⁺ is 1.750 μg/m³ or        less, and a total concentration of F⁻, Cl⁻, NO₂ ⁻, PO₄ ³⁻, Br⁻,        NO₃ ⁻, and SO₄ ²⁻ is 0.552 μg/m³ or less.

(2) An SPV measurement apparatus for measuring a concentration of Fe ina p-type silicon wafer based on measurement by an SPV method, theapparatus comprising:

a measurement stage on which the p-type silicon wafer is placed in theSPV measurement;

an optical module by which the p-type silicon wafer is illuminated withlight;

a probe used to measure a capacitance formed between a capacitive sensorprovided on a front end of the probe and a surface of the p-type siliconwafer;

a lock-in amplifier that amplifies and detects an SPV signalcorresponding to the capacitance measured with the probe;

a calibration chip for reducing measurement error;

a dissociation stage on which the p-type silicon wafer is placed whensubjected to a process of dissociating Fe—B pairs in the p-type siliconwafer;

a flash lamp for dissociating the Fe—B pairs in the p-type siliconwafer;

a robot arm transferring and delivering the p-type silicon wafer to andfrom the measurement stage and the dissociation stage;

a robot controller for controlling the robot arm;

a first housing receiving the measurement stage, the probe, and thecalibration chip;

a second housing receiving the optical module and the lock-in amplifier;

a third housing receiving the dissociation stage and the flash lamp; and

a fourth housing receiving the robot arm and the robot controller,

wherein a first chemical filter and a second chemical filter areprovided in upstream air flows of the first housing and the thirdhousing, respectively, and a total concentration of Na⁺, NH₄ ⁺, and K⁺and a total concentration of F⁻, Cl⁻, NO₂ ⁻, PO₄ ³⁻, Br, NO₃ ⁻, and SO₄²⁻ in an atmosphere inside each of the first housing and the thirdhousing are set to 1.750 μg/m³ or less and 0.552 μg/m³ or less,respectively.

(3) The SPV measurement apparatus according to (2) above, wherein athird chemical filter is provided in an upstream air flow of the fourthhousing, and a total concentration of Na⁺, NH₄ ⁺, and K⁺ and a totalconcentration of F⁻, Cl⁻, NO₂ ⁻, PO₄ ³⁻, Br, NO₃ ⁻, and SO₄ ²⁻ in anatmosphere inside the fourth housing are set to 1.750 μg/m³ or less and0.552 μg/m³ or less, respectively.

(4) The SPV measurement apparatus according to (2) above, wherein thefirst chemical filter and the second chemical filter are provided abovethe first housing and the third housing, respectively.

(5) The SPV measurement apparatus according to (3) above, wherein thethird chemical filter is provided above the fourth housing.

A second group of inventions of this disclosure completed based on theabove findings primarily includes the following features.

(6) A method of measuring a concentration of Fe in a p-type siliconwafer based on measurement by an SPV method,

wherein the measurement is performed in an atmosphere in which anorganic concentration measured by a wafer exposure test is 0.05 ng/cm²or less.

(7) An SPV measurement apparatus for measuring a concentration of Fe ina p-type silicon wafer based on measurement by an SPV method, theapparatus comprising:

a measurement stage on which the p-type silicon wafer is placed in theSPV measurement;

an optical module by which the p-type silicon wafer is illuminated withlight;

a probe used to measure a capacitance formed between a capacitive sensorprovided on a front end of the probe and a surface of the p-type siliconwafer;

a lock-in amplifier that amplifies and detects an SPV signalcorresponding to the capacitance measured with the probe;

a calibration chip for reducing measurement error;

a dissociation stage on which the p-type silicon wafer is placed whensubjected to a process of dissociating Fe—B pairs in the p-type siliconwafer;

a flash lamp for dissociating the Fe—B pairs in the p-type siliconwafer;

a robot arm transferring and delivering the p-type silicon wafer to andfrom the measurement stage and the dissociation stage;

a robot controller for controlling the robot arm;

a first housing receiving the measurement stage, the probe, and thecalibration chip;

a second housing receiving the optical module and the lock-in amplifier;

a third housing receiving the dissociation stage and the flash lamp; and

a fourth housing receiving the robot arm and the robot controller,

wherein a first chemical filter and a second chemical filter areprovided in upstream air flows of the first housing and the thirdhousing, respectively, and an inorganic concentration in an atmosphereinside each of the first housing and the third housing, measured by awafer exposure test, is set to 0.05 ng/cm² or less.

(8) The SPV measurement apparatus according to (7) above, wherein athird chemical filter is provided in an upstream air flow of the fourthhousing, and an inorganic concentration in an atmosphere inside thefourth housing, measured by a wafer exposure test, is set to 0.05 ng/cm²or less.

(9) The SPV measurement apparatus according to (7) above, wherein thefirst chemical filter and the second chemical filter are provided abovethe first housing and the third housing, respectively.

(10) The SPV measurement apparatus according to (8) above, wherein thethird chemical filter is provided above the fourth housing.

Advantageous Effect

The method of measuring the concentration of Fe in a p-type siliconwafer by an SPV method and the SPV measurement apparatus, according tothis disclosure can improve the measurement accuracy for Feconcentrations of 1×10⁹/cm³ or less.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a schematic view illustrating the structure of an SPVmeasurement apparatus 100 according to one embodiment of thisdisclosure;

FIG. 2 is a schematic view of part of the SPV measurement apparatus 100,illustrating the structure relating to the SPV measurement for measuringthe concentration of Fe in a p-type silicon wafer;

FIG. 3 is a graph illustrating the rate of change in the Signal valueobtained using a calibration chip in Experimental Example 1;

FIG. 4 is a graph illustrating the relationship between the average Feconcentration and the CV value in Example of Experimental Example 2,Comparative Example 1, and Comparative Example 2;

FIG. 5 is a graph illustrating the rate of change in the SPV signalvalue in Example of Experimental Example 2, Comparative Example 1, andComparative Example 2 when the center point of each p-type silicon waferwas measured once a day for ten days; and

FIG. 6 is a graph illustrating the rate of change in the SPV signalvalue in Example of Experimental Example 2 and Comparative Example 1when the center point of each p-type silicon wafer was measured once aday for 100 days.

DETAILED DESCRIPTION

One embodiment of this disclosure relates to a method of measuring theconcentration of Fe in a p-type silicon wafer based on the measurementof the Fe concentration in a silicon wafer by an SPV method (SPVmeasurement).

First, how to determine the Fe concentration at a certain portion in thesurface of the p-type silicon wafer will be described. Fe present in thep-type silicon wafer in a normal state combines with a dopant (forexample, boron) by electrostatic force to form Fe—B pairs. On the otherhand, when the wafer is illuminated with intense light, Fe becomesdissociated from B. The diffusion length of minority carriers,determined by SPV measurement means a distance over which the minoritycarriers generated by the light casted in the SPV measurement can move.The minority carriers are annihilated for example by being trapped by atrap level formed by Fe in the wafer. Trap levels formed by Fe in thep-type silicon wafer include Fe—B (iron-boron pairs) which is inherentlypresent and Fei (interstitial iron) formed by the light illumination.The trap levels formed by Fe in different forms have different minoritycarrier trapping abilities. Specifically, Fe can more easily trapminority carriers and the diffusion length is shorter in the dissociatedstate than in the normal state. Using the difference, the Feconcentration in the wafer can be found as follows.

First, SPV measurement is performed in a normal state and the diffusionlength L_(FeB) of minority carriers is determined. Next, SPV measurementis performed in a dissociated state and the diffusion length L_(Fei) ofminority carriers is determined. The Fe concentration [Fe] can becalculated by the following formula (1).

[Fe]=C×(1/L_(Fei) ²−1/L_(FeB) ²)  (1),

where C is a constant.

Thus, a map of the Fe concentration in the wafer can be obtained byperforming SPV measurements in the normal state and in the dissociatedstate on a plurality of portions in the wafer surface. The process fordissociating Fe—B pairs is by a usual method, for example, but notlimited to illumination using a flash lamp.

Next, referring to FIG. 2, an SPV measurement apparatus 100 according toone embodiment of this disclosure will be described, focusing on thestructure relating to SPV measurements. The SPV measurement apparatus100 has an optical module 10, a probe 18, a lock-in amplifier 20, and ameasurement stage 22. The optical module 10 has a light source 12, achopper 14, and a filter wheel 16.

The light source 12 is composed of for example white LEDs, and anoptical path is designed so that light emitted from the light source isdirected to the surface of a p-type silicon wafer W placed on themeasurement stage 22. The chopper 14 is a circular member having aplurality of openings in a circular pattern. The rotation of the chopperallows light emitted from the light source 12 to have a frequency. Inother words, the surface of the p-type silicon wafer W is illuminatedwith the light intermittently. The frequency of light obtained here isdefined as a “chopping frequency (CF)”, which is one of the measurementparameters. The CF is typically set to around 500 Hz to 3000 Hz.

The filter wheel 16 has the openings 16A to 16D provided with filtersthat transmit only lights having different wavelengths. This allows thesurface of the p-type silicon wafer W to be illuminated with lightshaving certain wavelengths.

Here, FIG. 2 illustrates a case where the optical module 10 is an analogmodule; alternatively, it may be a digital module. In the case where theoptical module 10 is a digital module, a plurality of single color LEDshaving different emission wavelengths are modularized, and the surfaceof the p-type silicon wafer W can be illuminated with lights havingcertain wavelengths with certain frequencies by making the LEDs flash.

The wavelengths of the illumination lights may be any differentwavelengths between 780 nm to 1004 nm. However, when SPV measurementsare performed using lights having two wavelengths, a combination of 780nm and 1004 nm can be given as an example of the wavelengths, and whenSPV measurements are performed using lights having four wavelengths, aset of 780 nm, 914 nm, 975 nm, and 1004 nm can be given as an example.

The intensity (amount) of illumination light is set as Injection Level,which is one of the measurement parameters. Typically, the amount oflight of Level 2 is 2×10¹² (atoms/cc), and that of Level 3 is 3×10¹²(atoms/cc), and one of these two parameters is used.

The probe 18 has a capacitive sensor on its front end, therebycontinuously measuring the capacitance formed between the surface of thep-type silicon wafer W and the probe 18. Prior to SPV measurement, thesurface of the p-type silicon wafer W is subjected to a HF treatment tobe positively charged. When the wafer W is illuminated with light fromthe light source 12, minority carriers (electrons for a p-type wafer)are generated in the wafer and migrate toward the positively chargedsurface. Upon reaching the surface, the electrons are annihilated by thepositive charges on the surface, so that the electric potential of thesurface decreases, resulting in reduced capacitance. The drop in thecapacitance is detected as an SPV signal. When more electrons aretrapped by Fe in the p-type silicon wafer, the surface potential is lesslikely to be reduced.

The lock-in amplifier 20 amplifies and detects an SPV signalcorresponding to the capacitance measured with the probe 18. Thus, anSPV signal can be obtained. Moving the measurement stage 22 allows SPVmeasurements to be performed on a plurality of portions on the surfaceof the p-type silicon wafer W.

The SPV apparatus may be a known SPV apparatus, such as for exampleFAaST 330 manufactured by Semilab-SDi LLC or SPV Station 1020manufactured by Strategic Diagnostics Inc.

Next, a method of an SPV measurement and how to determine the diffusionlength will be described. First, SPV measurement is performed usinglight having a first wavelength (for example, 780 nm) to obtain an SPVsignal corresponding to the light. In a graph, the “penetration length”dependent on the wavelength of the illumination light is represented bythe X-axis, and the “amount of light/SPV signal” is represented by theY-axis. The measurement results are plotted in the graph. Subsequently,an SPV measurement is performed using light having a second wavelength(for example, 1004 nm) different from the first wavelength to obtain anSPV signal corresponding to the light. Similarly, the measurementresults are plotted in the graph. The X intercept of the straight linejoining the thus obtained two plots can be determined as a “diffusionlength”. Note that when SPV measurements are performed using three ormore wavelengths, since three or more plots are obtained, the Xintercept is calculated by approximation such as the method of leastsquares.

Here, the measurement modes include two existing modes of Standard Modeand Ultimate Mode. In Standard Mode, an SPV measurement is performedusing a certain wavelength and sequentially other SPV measurements areperformed using wavelengths that are different from the precedingwavelength and from each other, so that the plots are obtainedsequentially. On the other hand, in Ultimate Mode, illumination with aplurality of lights having different wavelengths is performed by SPVmeasurements at a time, so that the plots are obtained by the one-timemeasurement. In this case, chopping frequencies of the wavelengths aremade different from each other, thereby obtaining SPV signals havingdifferent frequencies in the lock-in amplifier 20; thus, SPV signalscorresponding to the respective wavelengths can be obtained separately.In this embodiment, the measurement mode is not limited.

(First Group of Inventions)

Here, in this embodiment, it is important to perform an SPV measurementin an atmosphere in which a total concentration of Na⁺, NH₄ ⁺, and K⁺ is1.750 μg/m³ or less, and a total concentration of F⁻, Cl⁻, NO₂ ⁻, PO₄³⁻, Br⁻, NO₃ ⁻, and SO₄ ²⁻ is 0.552 μg/m³ or less. Accordingly, in thisembodiment, the measurement accuracy for Fe concentrations of 1×10⁹/cm³or less can be improved by reducing the ion concentration in the SPVmeasurement atmosphere.

The mechanism of such an effect is assumed as follows. First, foranions, as described above, in an SPV measurement on a p-type siliconwafer, the surface of the wafer is required to be positively passivatedby pretreatment such as HF cleaning. Here, when an acidic gas (anions)is in contact with the passivated (weakly charged) surface, the measuredvalue of the diffusion length varies. Further, when a measurement at anFe concentration of 1×10⁹/cm³ or less is performed, the differencebetween the diffusion lengths before and after dissociation is small(L_(FeB)≈L_(Fei)), thus the measurement variation in the diffusionlength has great influence. Accordingly, it is assumed that themeasurement of the diffusion length is stabilized by removing anions,and the quantitativity of the Fe—B energy level density can be improved.

Next, for cations, a calibration chip is integrated in or mounted on anSPV measurement apparatus to calibrate the apparatus, and this chip isbuilt in an n-type silicon wafer. The surface of the n-type siliconwafer is negatively passivated as opposed to the p-type silicon wafer.When an alkaline gas (cations) is in the measurement atmosphere, themeasurement accuracy of the calibration chip is poor and this affectsthe calibration in the diffusion length measurement. Accordingly, it isassumed that the absolute value of the measurement of the diffusionlength is ensured by removing cations, and the quantitativity of theFe—B energy level density can be improved.

In the silicon wafer production process, sodium hydroxide or potassiumhydroxide is chiefly used as an alkaline etchant, and nitrichydrofluoric acid is chiefly used as an acidic etchant. Bromic acid saltmay be added to the alkaline etchant, and phosphoric acid or sulfuricacid may be added to the acidic etchant. Further, aqueous ammonium,hydrochloric acid, and hydrogen peroxide are chiefly used as cleaningsolutions for wafers. It is only necessary to consider, as gasesgenerated in the use of those chemicals, three types of cations: Na⁺,NH₄ ⁺, and K⁺ and seven types of anions: F⁻, Cl⁻, NO₂ ⁻, PO₄ ³⁻, Br⁻,NO₃ ⁻, and SO₄ ²⁻. Further, although the external air contains ions suchas Na⁺ and Cl⁻ from salt wind; however, this does not matter as long asthe ions mentioned above are monitored.

By way of example, an aspect of controlling the ion concentration in theSPV measurement atmosphere is controlling the ion concentration asdescribed above in an environment in which the SPV measurement apparatusis placed. Specifically, in a clean room in which the SPV measurementapparatus is placed, chemical filters such as a cation filter forremoving cations and an anion filter for removing anions are placed soas to control the ion concentration as described above in the atmospherein the clean room. Examples of the cation filter include PureLitePF590F4H manufactured by Nippon Puretec Co., Ltd. and PL-C-25-4 GImanufactured by Dan-Takuma Technologies Inc. Examples of the anionfilter include PureLite P592E5H manufactured by Nippon Puretec Co., Ltd.and PL-A-30-4 GO manufactured by Dan-Takuma Technologies Inc. The placein which the chemical filters are placed in the clean room may bedetermined as appropriate in terms of suitably reducing the ionconcentration in the atmosphere in the clean room. The air inside aclean room usually consists of circulated air and the external air takenin to compensate for pressure loss. The circulated air is purifiedthrough a HEPA filter placed midway through (preferably on the ceilingof the clean room) the air flow formed by a circulation fan, andintroduced into the clean room. The clean room is also designed to guidethe air introduced through an external air inlet to the circulation fan.Accordingly, the chemical filters are preferably placed on the externalair inlet and also between the circulation fan and the HEPA filter.

Another aspect of controlling the ion concentration in the SPVmeasurement atmosphere is controlling the ion concentration as describedabove in the atmosphere of a certain space in the SPV measurementapparatus 100 used for the measurement. This aspect will be describedbelow with reference to FIG. 1.

The SPV measurement apparatus 100 is segmented into a plurality ofspaces using a plurality of housings. A first housing 38 receives themeasurement stage 22 and the probe 18 that have already been describedwith reference to FIG. 2, and a calibration chip 24 for reducingmeasurement error. A second housing 40 receives the optical module 10and the lock-in amplifier 20 that have already been described withreference to FIG. 2. A third housing 42 receives a dissociation stage 26on which a p-type silicon wafer is placed when Fe—B pairs in the p-typesilicon wafer are dissociated, and a flash lamp 28 for dissociating theFe—B pairs in the p-type silicon wafer. A fourth housing 44 receives arobot arm 30 for transferring and delivering the p-type silicon wafer toand from the measurement stage 22 and the dissociation stage 26, a robotcontroller 32 for controlling the robot arm 30, and an aligner 34 foraligning the position of a notch of the p-type silicon wafer. A fifthhousing 46 receives a control computer 36 for controlling the entireapparatus.

The p-type silicon wafer W is transferred as described below andsubjected to an SPV measurement. First, a plurality of p-type siliconwafers W received in a load port not shown are put on the robot arm 30provided in the fourth housing 44 one by one, and the notch of eachwafer is aligned by the aligner 34. Next, the wafer is transferred intothe first housing 38 by the robot arm 30 and placed on the measurementstage 22. Next, the wafer is subjected to an SPV measurement in a normalstate on the measurement stage 22. Next, the wafer is delivered from thefirst housing 38 by the robot arm 30, transferred into the third housing42, and placed on the dissociation stage 26. Next, the wafer issubjected to dissociation treatment by being illuminated on thedissociation stage 26 by the flash lamp 28, resulting in a dissociationstate. Next, the wafer is delivered from the third housing 42 by therobot arm 30, transferred back into the first housing 38, and placed onthe measurement stage 22. Next, the wafer is subjected to an SPVmeasurement in the dissociation state on the measurement stage 22.Finally, the wafer is delivered from the first housing 38 by the robotarm 30, unloaded from the SPV measurement apparatus 100, and returned tothe load port.

Here, it is important that a first chemical filter 48 and a secondchemical filter 50 are provided in upstream air flows of the firsthousing 38 and the third housing 42, respectively, and the totalconcentration of Na⁺, NH₄ ⁺, and K⁺ and the total concentration of F⁻,Cl⁻, NO₂ ⁻, PO₄ ³⁻, Br⁻, NO₃ ⁻, and SO₄ ²⁻ in an atmosphere inside eachof the first housing 38 and the third housing 42 are set to 1.750 μg/m³or less and 0.552 μg/m³ or less, respectively. In the SPV measurement,at least the atmosphere in the first housing 38 where the measurement isactually performed and the atmosphere of the third housing 42 where thedissociation treatment is performed are required to be controlled.Controlling the atmosphere in the first and third housings as describedabove would not affect the measured values, since ions are not depositedon the measurement stage 22 or the dissociation stage 26 and are notattached to the back surface of the wafer. For the first chemical filter48 and the second chemical filter 50, the cation filters and the anionfilters mentioned above can suitably be used.

Further, in terms of further improving measurement accuracy, preferably,a third chemical filter 52 is provided in an upstream air flow of thefourth housing 44 to also control the atmosphere in the fourth housing44 as described above. This further improves the measurement accuracy,since ions are not deposited on the robot arm 30 in the fourth housing44 and are not attached to the back surface of the wafer.

It should be noted that the first housing 38, the third housing 42, andthe fourth housing 44 may be integrated to segment one measurement space(into a measurement area, a dissociation treatment area, and a transferarea).

Further, to prevent turbulence from being caused in the air flow aspossible, the first, second, and third chemical filters 48, 50, 52 arepreferably provided above the first, third, and fourth housings 38, 42,44, respectively.

(Second Group of Inventions)

Here, in this embodiment, it is important to perform an SPV measurementin an atmosphere in which the organic concentration measured by a waferexposure test is 0.05 ng/cm² or less. Accordingly, in this embodiment,the measurement accuracy for the Fe concentrations of 1×10⁹/cm³ or lesscan be improved by reducing the organic concentration in the SPVmeasurement atmosphere.

The mechanism of such an effect is assumed as follows. As describedabove, in an SPV measurement on a p-type silicon wafer, the surface ofthe wafer is required to be positively passivated by pretreatment suchas HF cleaning. Here, when an organic material adheres the passivated(weakly charged) surface, weak charge results in varied measured valuesof the diffusion length. Further, when a measurement at an Feconcentration of 1×10⁹/cm³ or less is performed, the difference betweenthe diffusion lengths before and after dissociation is small(L_(FeB)≈L_(Fei)), thus the measurement variation in the diffusionlength has great influence. Accordingly, it is assumed that themeasurement of the diffusion length is stabilized by removing organicmaterials, and the quantitativity of the Fe—B energy level density canbe improved.

In the silicon wafer production process, an alcohol-based solvent suchas isopropyl alcohol (IPA) may be used after the wafer cleaning.Further, particularly in a process of producing an epitaxial wafer withan embedded diffusion layer, since photo resist is used,hexamethyldisilazane (HMDS) for improving the adhesion of the resist, anorganic liquid such as tetramethylammonium hydroxide (TMAH) as adeveloper, etc. are used. Further, the external air may contain organicmaterials in the exhausts from factories including the relevant factory,and such organic materials are brought into the clean room when theexternal air is taken into the clean room. To address this, in thisembodiment, the measurement accuracy for Fe concentrations of 1×10⁹/cm³or less is improved by removing those organic materials.

By way of example, an aspect of controlling the organic concentration inthe SPV measurement atmosphere is controlling the organic concentrationas described above in an environment in which the SPV measurementapparatus is placed. Specifically, in a clean room in which the SPVmeasurement apparatus is placed, chemical filters for removing organicmaterials is placed so as to control the organic concentration asdescribed above in the atmosphere in the clean room. Examples of thechemical filters for removing organic materials include PureLite PF592FN(MAF) manufactured by Nippon Puretec Co., Ltd. The place in which thechemical filters are placed in the clean room may be determined asappropriate in terms of suitably reducing the organic concentration inthe atmosphere in the clean room. The chemical filters are preferablyplaced on the external air inlet and also between the circulation fanand the HEPA filter, as in the first group of inventions.

Another aspect of controlling the organic concentration in the SPVmeasurement atmosphere is controlling the organic concentration asdescribed above in the atmosphere of a certain space in the SPVmeasurement apparatus 100 used for the measurement. For the descriptionof the basic structure of the SPV measurement apparatus 100 withreference to FIG. 1, hereinafter refer to the description of the firstgroup of inventions.

Here, it is important that a first chemical filter 48 and a secondchemical filter 50 are provided in upstream air flows of the firsthousing 38 and the third housing 42, respectively, and the concentrationof organic materials measured by a wafer exposure test is set to 0.05ng/cm² or less in the first housing 38 and the third housing 42. In theSPV measurement, at least the atmosphere in the first housing 38 wherethe measurement is actually performed and the atmosphere of the thirdhousing 42 where the dissociation treatment is performed are required tobe controlled. Controlling the atmosphere in the first and thirdhousings as described above would not affect the measured values, sinceorganic materials are not deposited on the measurement stage 22 or thedissociation stage 26 and are not attached to the back surface of thewafer. For the first chemical filter 48 and the second chemical filter50, the chemical filters mentioned above can suitably be used.

Further, in terms of further improving measurement accuracy, preferably,a third chemical filter 52 is provided in an upstream air flow of thefourth housing 44 to also control the atmosphere in the fourth housing44 as described above. This further improves the measurement accuracy,since organic materials are not deposited on the robot arm 30 in thefourth housing 44 and are not attached to the back surface of the wafer.

It should be noted that the first housing 38, the third housing 42, andthe fourth housing 44 may be integrated to segment one measurement space(into a measurement area, a dissociation treatment area, and a transferarea).

Further, to prevent turbulence from being caused in the air flow aspossible, the first, second, and third chemical filters 48, 50, 52 arepreferably provided above the first, third, and fourth housings 38, 42,44, respectively.

EXAMPLES Experimental Example 1

Experimental Example 1 relating to the first group of inventions isdescribed below.

Example

An SPV measurement apparatus (FAaST 330 (digital) manufactured bySemilab-SDi LLC) was placed in a clean room. In the relevant clean room,an anion filter (PureLite P592E5H manufactured by Nippon Puretec Co.,Ltd.) and a cation filter (PureLite PF590F4H manufactured by NipponPuretec Co., Ltd.) were newly provided to reduce the ion concentration.Table 1 shows the ion concentration measured by the following method.Note that Nos. 1 to 6 correspond to measurements performed on differentdays. Environmental conditions other than the ion concentration weretemperature: 24±2° C., relative humidity: 30% to 50%, and cleanliness:class 7 (JIS standards) as recommended by the manufacturer.

<Ion Concentration Measurement Method>

The ion concentration was measured by a pure water impinger bubblingmethod.

-   -   Pure water: 100 mL    -   Suction speed: 1 L/min    -   Suction time: 360 min    -   Analyzer: ion chromatography

The conversion into the ion concentration in an atmosphere of 1 m³ wasperformed using the following conversion formula.

Value found by the analysis (analysis value) [ppb]×amount of pure waterrecovered [mL]/suction amount [m³]=ion concentration [μg/m³]

Of total nine p-type silicon wafers prepared; three p-type siliconwafers had an Fe concentration of the order of 10⁸/cm³, three p-typesilicon wafers had an Fe concentration in the first half of the order of10⁹/cm³, and three p-type silicon wafers had an Fe concentration in thesecond half of the order of 10⁹/cm³ to the order of 10¹⁰/cm³. In theatmospheres corresponding to Nos. 1 to 6 in Table 1, the Feconcentration of 177 points in the surface of each wafer was measuredthree times using the above SPV measurement apparatus. The measurementconditions were set as recommended by the manufacturer, and theillumination wavelength was set to 780 nm and 1004 nm.

<Evaluation of Measurement Accuracy>

For each wafer, variations of the three average Fe concentrations of 177points in the surface (variance of the three average Feconcentrations/average of the three average Fe concentrations×100) aregiven as CV values in Table 2. The CV value is preferably 10% or less.

Further, the number of undefined points where L_(Fei)>L_(FeB) in thevalues measured with respect to the 177 points in each wafer wascounted, and the average of the three average values is shown as anaverage UD value in Table 3.

In Table 2 and Table 3, the wafers having an Fe concentration of theorder the of 10⁸/cm³ are represented as “WFs 1 to 3”, the wafers havingan Fe concentration in the first half of the order of 10⁹/cm³ arerepresented as “WFs 4 to 6”, and the wafers having an Fe concentrationin the second half of the order of 10⁹/cm³ to the order of 10¹⁰/cm³ arerepresented as “WFs 7 to 9”. The average value of the three average Feconcentrations measured for each wafer in Example is given in Table 4.

Further, aside from the above SPV measurements, measurements of Signalvalues using a calibration chip was performed every day for one month,and the rate of change was found, given the Signal value of the firstday was 1. The results are given in FIG. 3.

Comparative Example 1

Measurements of ion concentrations and evaluation of the measurementaccuracy were performed in the same manner as in Example except that nochemical filter was provided in the clean room and so the ionconcentration was not reduced. The results are given in Tables 1 to 3and FIG. 3.

Comparative Example 2

Measurements of ion concentrations and evaluation of the measurementaccuracy were performed in the same manner as in Example using an SPVmeasurement apparatus of the same model as in Example, provided in aclean room of a different factory from Comparative Example 1, in whichclean room no chemical filter was provided. The results are given inTables 1 to 3 and FIG. 3.

TABLE 1 Cation Anion Category No. Na⁺ NH₄ ⁺ K⁺ F⁻ Cl⁻ NO₂ ⁻ PO₄ ³⁻ Br⁻NO₃ ⁻ SO₄ ²⁻ Example 1 <0.002 1.143 <0.003 <0.006 <0.002 <0.500 <0.017<0.011 <0.008 <0.008 2 <0.002 1.744 <0.003 <0.006 <0.002 <0.500 <0.017<0.011 <0.008 <0.008 3 <0.002 1.383 0.007 <0.006 <0.002 <0.500 <0.017<0.011 <0.008 <0.008 4 <0.002 1.143 <0.003 <0.006 <0.002 <0.500 <0.017<0.011 <0.008 <0.008 5 <0.002 1.289 0.005 <0.006 <0.002 <0.500 <0.017<0.011 <0.008 <0.008 6 <0.002 1.444 <0.003 <0.006 <0.002 <0.500 <0.017<0.011 <0.008 <0.008 Comparative 1 <0.002 2.067 <0.003 0.026 0.052<0.500 0.017 0.011 0.440 0.017 Example 1 Comparative 1 0.018 7.596 0.0250.010 0.012 0.748 0.017 0.011 0.020 0.013 Example 2 2 0.017 4.869 0.0860.007 0.013 1.567 0.017 0.011 0.035 0.009 3 0.017 5.074 0.025 0.0080.010 1.698 0.017 0.011 0.064 0.013 4 0.017 5.401 0.025 0.006 0.0161.688 0.017 0.011 0.092 0.021 5 0.017 5.074 0.025 0.008 0.010 1.6980.017 0.011 0.064 0.013 6 0.017 5.991 0.025 0.019 0.013 2.136 0.0170.011 0.030 0.023 7 0.017 6.005 0.025 0.006 0.018 1.537 0.017 0.0110.057 0.025 8 0.017 6.310 0.025 0.006 0.017 0.981 0.017 0.011 0.0400.009

TABLE 2 CV value (%) Category WF1 WF2 WF3 WF4 WF5 WF6 WF7 WF8 WF9Example 3.0% 5.8% 3.0% 2.2% 1.9% 1.4% 0.0% 1.3% 1.1% 4.7% 7.9% 3.5% 1.8%1.2% 1.4% 1.1% 0.9% 0.6% 4.9% 3.8% 8.8% 0.8% 1.0% 0.8% 0.4% 0.3% 0.6%5.4% 9.6% 5.5% 0.8% 1.3% 1.7% 0.7% 1.6% 0.3% 7.9% 4.8% 6.9% 1.4% 1.8%1.1% 0.8% 0.4% 0.3% 5.0% 8.8% 6.0% 2.2% 2.1% 1.2% 0.9% 0.7% 0.8%Comparative 12.1% 15.3% 18.4% 2.1% 1.0% 0.5% 0.5% 2.8% 1.4% Example 1Comparative 16.7% 28.8% 19.6% 4.6% 2.9% 1.8% 1.1% 2.0% 1.2% Example 224.7% 10.9% 28.1% 4.9% 2.1% 3.9% 1.8% 1.3% 1.6% 19.2% 17.9% 15.7% 3.8%4.8% 0.4% 1.2% 1.7% 0.9% 27.9% 11.2% 13.1% 3.0% 5.8% 3.0% 2.2% 1.9% 1.4%19.0% 12.8% 30.4% 4.3% 6.3% 5.9% 0.8% 1.8% 1.4% 57.7% 29.5% 24.1% 5.9%2.8% 4.8% 0.4% 1.2% 1.7% 13.3% 24.7% 10.9% 3.9% 4.7% 1.1% 1.1% 1.3% 0.8%14.8% 21.5% 14.1% 3.9% 2.8% 1.6% 1.2% 1.4% 3.0%

TABLE 3 Average UD (number) Category WF1 WF2 WF3 WF4 WF5 WF6 WF7 WF8 WF9Example 0 5 1 0 0 0 0 0 0 3 5 2 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 00 0 0 0 0 1 0 0 0 0 0 0 0 2 3 0 0 0 0 0 0 0 Comparative 16 6 10 0 0 0 00 0 Example 1 Comparative 23 10 22 1 0 0 0 0 0 Example 2 14 33 21 0 0 00 0 1 12 30 4 1 0 0 0 0 0 29 23 6 2 0 0 0 0 0 38 31 55 1 0 1 0 0 0 41 1510 8 10 2 0 1 1 16 6 14 6 4 0 1 0 1 10 13 7 0 0 0 0 0 0

TABLE 4 WF Average Fe concentration (/cm³) WF1 8.5.E+08 WF2 7.7.E+08 WF37.8.E+08 WF4 4.1.E+09 WF5 4.3.E+09 WF6 4.2.E+09 WF7 1.4.E+10 WF86.6.E+09 WF9 9.3.E+09

<Evaluation Results>

As evident from Table 2 and Table 3, the CV value was made as low as 10%or less and the average UD value was minimized to almost zero in Exampleeven when the Fe concentration was 1×10⁹/cm³ or less. On the other hand,in Comparative Examples 1, 2, the CV value exceeded 10% and the averageUD value was also high at an Fe concentration of 1×10⁹/cm³ or less.Further, as evident from FIG. 3, change in the Signal value was small inExample. On the other hand, change in the Signal value was large inComparative Examples 1, 2; the amount of reduction in the Signal valuewas especially large in Comparative Example 2 where the number ofcations was large.

Experimental Example 2

Experimental Example 2 relating to the second group of inventions isdescribed below.

Example

An SPV measurement apparatus (FAaST 330 (digital) manufactured bySemilab-SDi LLC) was placed in a clean room. In the relevant clean room,a chemical filter for removing organic materials (PureLite PF-592FN(MAF) manufactured by Nippon Puretec Co., Ltd.) was provided to reducethe organic concentration. Table 5 shows the organic concentrationmeasured by the following method. Environmental conditions other thanthe organic concentration were temperature: 24±2° C., relative humidity:30% to 50%, and cleanliness: class 7 (JIS standards) as recommended bythe manufacturer.

<Organic Concentration Measurement Method>

The organic concentration was measured by the wafer exposure test asdescribed below. A silicon wafer having a diameter of 300 mm was exposedin a clean room atmosphere for 5 hours. After that, the exposed waferwas heated, and the mass of the whole liberated gas was analyzed byGC-MS. The analysis value (ng) obtained was converted to a value per thewafer area (cm²) as an organic concentration (ng/cm²).

Of total nine p-type silicon wafers prepared; three p-type siliconwafers had an Fe concentration of the order of 10⁸/cm³, three p-typesilicon wafers had an Fe concentration in the first half of the order of10⁹/cm³, and three p-type silicon wafers had an Fe concentration in thesecond half of the order of 10⁹/cm³ to the order of 10¹⁰/cm³. In theatmospheres of the organic concentrations given in Table 1, the Feconcentration of 177 points in the surface of each wafer was measuredthree times using the above SPV measurement apparatus. The measurementconditions were set as recommended by the manufacturer, and theillumination wavelength was set to 780 nm and 1004 nm.

<Evaluation of Measurement Accuracy>

For each wafer, variations of the three average Fe concentrations of 177points in the surface (variance of the three average Feconcentrations/average of the three average Fe concentrations×100) werefound as CV values. FIG. 4 presents a graph in which the average of thethree average Fe concentrations in each wafer is plotted on thehorizontal axis, and the CV value is plotted on the vertical axis. TheCV value is preferably 10% or less.

Further, aside from the above SPV measurements, the rate of change inthe Signal value of one center point of each Fe concentration of theorder of 10⁸/cm³ measured repeatedly once a day for ten days is given inFIG. 5, given the measured value of the first time was 1. Further, inorder to further ascertain the change in the SPV signal values, themeasurement was continued to the one hundredth day in Example andComparative Example 1, and the rate of change is given in FIG. 6, giventhe measured value of the first time day was 1, as in FIG. 5.

Comparative Example 1

Evaluation of the measurement accuracy was performed in the same manneras in Example except for the environment in which no chemical filter wasprovided, little external air was taken into the clean room, andcirculation was increased. The results are given in Table 5 and FIGS. 4to 6.

Comparative Example 2

Evaluation of the measurement accuracy was performed in the same manneras in Example except for the environment in which no chemical filter wasprovided, a large amount of external air was taken into the clean room,and circulation was reduced. The results are given in Table 5 and FIGS.4 to 5.

TABLE 5 Category Organic concentration (ng/cm²) Example 0.05 ComparativeExaxmple 1 0.08 Comparative Exaxmple 2 0.42

<Evaluation Results>

As evident from FIG. 4, in Example in which less organic materials werepresent in the atmosphere, the CV value was made as low as 10% or lesseven at an Fe concentration of 1×10⁹/cm³ or less. On the other hand, inComparative Examples 1, 2 in which rather more organic materials werepresent in the atmosphere, the CV value exceeded 10% at an Feconcentration of 1×10⁹/cm³ or less. This demonstrates that when the Feconcentration is 1×10⁹/cm³ or less, in an environment in which theconcentration of organic materials measured by the wafer exposure testis more than 0.05 ng/cm², the reproducibility of repeated measurementsis poor.

Further, FIG. 5 demonstrates that the value of the SPV signal wasreduced each time the measurement was performed in Comparative Example2, FIG. 6 demonstrates that the value of the SPV signal was graduallyreduced in a long term also in the environment in the environment ofComparative Example 1, and the reduction reached as high as 25% onehundred days later. On the other hand, no reduction in the value of theSPV signal was observed. The above results indicate that if theenvironment contains a large amount of organic materials, themeasurement intensity is low even when the same measurements areperformed, thus the reliability of the measured values is low. Thus, inthe environment containing a large amount of organic materials, theorganic materials annihilate surface charge or produce electrical noise,which is considered to affect electrical signals of the SPVmeasurements.

INDUSTRIAL APPLICABILITY

The method of measuring the concentration of Fe in a p-type siliconwafer by an SPV method and the SPV measurement apparatus, according tothis disclosure improves the measurement accuracy for Fe concentrationsof 1×10⁹/cm³ or less.

REFERENCE SIGNS LIST

-   -   100: SPV measurement apparatus    -   10: Optical module    -   12: Light source    -   14: Chopper    -   16: Filter wheel    -   18: Probe    -   20: Lock-in amplifier    -   22: Measurement stage    -   24: Calibration chip    -   26: Dissociation stage    -   28: Flash lamp    -   30: Robot arm    -   32: Robot controller    -   34: Aligner    -   36: Control computer    -   38: First housing    -   40: Second housing    -   42: Third housing    -   44: Fourth housing    -   46: Fifth housing    -   48: First chemical filter    -   50: Second chemical filter    -   52: Third chemical filter    -   W: P-type silicon wafer

1. A method of measuring a concentration of Fe in a p-type silicon waferbased on measurement by an SPV method, wherein the measurement isperformed in an atmosphere in which a total concentration of Na⁺, NH₄ ⁺,and K⁺ is 1.750 μg/m³ or less, and a total concentration of F⁻, Cl⁻, NO₂⁻, PO₄ ³⁻, Br⁻, NO₃ ⁻, and SO₄ ²⁻ is 0.552 μg/m³ or less.
 2. An SPVmeasurement apparatus for measuring a concentration of Fe in a p-typesilicon wafer based on measurement by an SPV method, the apparatuscomprising: a measurement stage on which the p-type silicon wafer isplaced in the SPV measurement; an optical module by which the p-typesilicon wafer is illuminated with light; a probe used to measure acapacitance formed between a capacitive sensor provided on a front endof the probe and a surface of the p-type silicon wafer; a lock-inamplifier that amplifies and detects an SPV signal corresponding to thecapacitance measured with the probe; a calibration chip for reducingmeasurement error; a dissociation stage on which the p-type siliconwafer is placed when subjected to a process of dissociating Fe—B pairsin the p-type silicon wafer; a flash lamp for dissociating the Fe—Bpairs in the p-type silicon wafer; a robot arm transferring anddelivering the p-type silicon wafer to and from the measurement stageand the dissociation stage; a robot controller for controlling the robotarm; a first housing receiving the measurement stage, the probe, and thecalibration chip; a second housing receiving the optical module and thelock-in amplifier; a third housing receiving the dissociation stage andthe flash lamp; and a fourth housing receiving the robot arm and therobot controller, wherein a first chemical filter and a second chemicalfilter are provided in upstream air flows of the first housing and thethird housing, respectively, and a total concentration of Na⁺, NH₄ ⁺,and K⁺ and a total concentration of F⁻, Cl⁻, NO₂ ⁻, PO₄ ³⁻, Br⁻, NO₃ ⁻,and SO₄ ²⁻ in an atmosphere inside each of the first housing and thethird housing are set to 1.750 μg/m³ or less and 0.552 μg/m³ or less,respectively.
 3. The SPV measurement apparatus according to claim 2,wherein a third chemical filter is provided in an upstream air flow ofthe fourth housing, and a total concentration of Na⁺, NH₄ ⁺, and K⁺ anda total concentration of F⁻, Cl⁻, NO₂ ⁻, PO₄ ³⁻, Br⁻, NO₃ ⁻, and SO₄ ²⁻in an atmosphere inside the fourth housing are set to 1.750 μg/m³ orless and 0.552 μg/m³ or less, respectively.
 4. The SPV measurementapparatus according to claim 2, wherein the first chemical filter andthe second chemical filter are provided above the first housing and thethird housing, respectively.
 5. The SPV measurement apparatus accordingto claim 3, wherein the third chemical filter is provided above thefourth housing.
 6. A method of measuring a concentration of Fe in ap-type silicon wafer based on measurement by an SPV method, wherein themeasurement is performed in an atmosphere in which an organicconcentration measured by a wafer exposure test is 0.05 ng/cm² or less.7. An SPV measurement apparatus for measuring a concentration of Fe in ap-type silicon wafer based on measurement by an SPV method, theapparatus comprising: a measurement stage on which the p-type siliconwafer is placed in the SPV measurement; an optical module by which thep-type silicon wafer is illuminated with light; a probe used to measurea capacitance formed between a capacitive sensor provided on a front endof the probe and a surface of the p-type silicon wafer; a lock-inamplifier that amplifies and detects an SPV signal corresponding to thecapacitance measured with the probe; a calibration chip for reducingmeasurement error; a dissociation stage on which the p-type siliconwafer is placed when subjected to a process of dissociating Fe—B pairsin the p-type silicon wafer; a flash lamp for dissociating the Fe—Bpairs in the p-type silicon wafer; a robot arm transferring anddelivering the p-type silicon wafer to and from the measurement stageand the dissociation stage; a robot controller for controlling the robotarm; a first housing receiving the measurement stage, the probe, and thecalibration chip; a second housing receiving the optical module and thelock-in amplifier; a third housing receiving the dissociation stage andthe flash lamp; and a fourth housing receiving the robot arm and therobot controller, wherein a first chemical filter and a second chemicalfilter are provided in upstream air flows of the first housing and thethird housing, respectively, and an inorganic concentration in anatmosphere inside each of the first housing and the third housing,measured by a wafer exposure test, is set to 0.05 ng/cm² or less.
 8. TheSPV measurement apparatus according to claim 7, wherein a third chemicalfilter is provided in an upstream air flow of the fourth housing, and aninorganic concentration in an atmosphere inside the fourth housing,measured by a wafer exposure test, is set to 0.05 ng/cm² or less.
 9. TheSPV measurement apparatus according to claim 7, wherein the firstchemical filter and the second chemical filter are provided above thefirst housing and the third housing, respectively.
 10. The SPVmeasurement apparatus according to claim 8, wherein the third chemicalfilter is provided above the fourth housing.